as president obama made clear today, there's still no evidence of who is responsible for this act of terrorism. >> this was a heinous and cowardly act, and given what we now know about what took place, the fbi is investigating it as an act of terrorism. >> reporter: the fbi agents tell us they have tons of video and somewhere in there will be a picture of the person who planted these two bombs, diane. >> speaking of pictures, i want to ask you about a photo that everybody was e-mailing today. it is a man standing up on top of a roof. you can move in there, you see it right there. what do you know about it? >> reporter: well, it's very intriguing, but authorities say they're giving it no special priority. it's one of many tips and leads they're following. they're really focusing on the person who had that unusually heavy black bag the day of theerates. >> as you said the strongest clues are in the nature of the bomb. up next, abc's jim avila shows us just how far the blast from the bombs traveled. >> reporter: the killer bombs likely contained less than two pounds of common black powd